Field of the Invention
The present invention relates to a numerical controller, and more particularly, to a numerical controller capable of ensuring the machining accuracy of a workpiece (object to be machined) while suppressing shock on a machine, by calculating a curve in consideration of an inward turning amount due to post-interpolation acceleration/deceleration and inserting the curve into a command path of a machining program.
Description of the Related Art
In continuously executing blocks for a machining command in numerical control, a corner is formed in a machining path as the movement direction changes. Since the speed of each moving axis suddenly changes at a corner section, a machine is liable to be shocked.
In conventional numerical control, acceleration/deceleration processing for each axis is performed near the corner to suppress such shock. The acceleration/deceleration processing includes pre-interpolation acceleration/deceleration and post-interpolation acceleration/deceleration. In the pre-interpolation acceleration/deceleration, acceleration/deceleration is performed along the machining path before interpolation processing. In the post-interpolation acceleration/deceleration, acceleration/deceleration is performed for each axis after the interpolation processing.
In the pre-interpolation acceleration/deceleration, a corner speed is calculated such that a speed variation of each axis is not larger than a predetermined allowable speed difference, in order to suppress a sudden speed change of each axis at the corner section. Then, speed control is performed in such a manner that the feed speed is reduced at a point short of the corner section so that the speed at the corner section reaches the calculated corner speed and that the feed speed is increased after the corner section is reached.
In the post-interpolation acceleration/deceleration, the speeds of the axes determined by the post-interpolation acceleration/deceleration are locally averaged on time basis, that is, control is made to suppress a speed change of each axis, in order to further suppress shock on the machine. Consequently, acceleration/deceleration is performed overlapping between the blocks, so that the machining path is deviated from a specified path, thereby causing an inward turning error.
Instead of suppressing the shock on the machine by the acceleration/deceleration processing, the corner itself may be removed by changing the command. In this method, the corner is removed by inserting a curve that smoothly connects blocks ahead of and behind the corner within the range of an allowable inward turning amount into the corner section in the machining command. This method is the same as the method of post-interpolation acceleration/deceleration in that the inward turning occurs. Since the post-interpolation acceleration/deceleration is averaged on time basis, however, the inward turning amount changes depending on the corner shape and the feed speed. Since the curve inserted into the corner section does not depend on the corner shape or the feed speed, in contrast, the machining accuracy can be easily controlled.
Japanese Patent Application Laid-Open No. 2008-225825 has proposed a prior art method for controlling the inward turning amount at the corner section. According to this method, a corner is removed by inserting a plurality of smooth curves between blocks on its opposite sides. Further, a technique disclosed in Japanese Patent Application Laid-Open No. 2013-069123 has solved the problem of the inward turning amount exceeding its tolerance on account of post-interpolation acceleration/deceleration by reducing a target feed speed at the corner section to an appropriate speed.
Furthermore, Japanese Patent Application Laid-Open No. 2014-021759 has proposed a method in which an inward turning amount at a corner is controlled to be within a predetermined tolerance without regard to the machining shape or the command speed by inserting a plurality of curves between blocks ahead of and behind the corner, and shock on a machine is eased by continuously varying the direction and curvature at the corner.
When post-interpolation acceleration/deceleration is performed, the inward turning amount changes depending on the direction changing angle at the corner section, the difference between axes moving on the opposite sides of the corner, or a time constant indicative of post-interpolation acceleration/deceleration properties. In order to restrict the inward turning amount to a certain value, therefore, the allowable speed difference between the axes and the time constant of the post-interpolation acceleration/deceleration must be adjusted for each machine and machining program used.
Although the inward turning amount at the corner section can be controlled when the curves are inserted into the corner section, on the other hand, interpolation points become discrete data as the curves are interpolated. Since command pulses for a servomotor are generated from the interpolation points, they cannot take strictly continuous values. Thus, it is difficult to fully suppress shock on the machine caused by the insertion of the curves, so that post-interpolation acceleration/deceleration is generally performed in conjunction with the insertion.
Since the servomotor is also subject to some delay from the command pulses from a numerical controller, moreover, influences of the post-interpolation acceleration/deceleration and the servomotor delay cannot be completely ignored even when the curves are inserted into the corner section.
However, the techniques described in Japanese Patent Applications Laid-Open Nos. 2008-225825 and 2014-021759 have a problem that the curves are inserted into the corner section not taking into consideration the inward turning amount due to the post-interpolation acceleration/deceleration or the servomotor delay. Further, the technique described in Japanese Patent Application Laid-Open No. 2013-069123 has a problem that the time constant of the post-interpolation acceleration/deceleration has to be increased to suppress the shock at the corner section, so that the feed speed has to be fully reduced to limit the inward turning amount at the corner section.